Since the recent communication network is developed to have a large amount of information and the communication network comes to have-variety of forms, a system of FTTH (Fiber to The Home) is going to be realized, whereby optical fiber networks are built up to the neighborhood of each home so that a great amount of information is handled and transmitted at a high speed. FIG. 61 is a diagram showing one mode of communication network, or an ATM-PON 100, for example. Recently, with regard to the ATM-PON 100, a recommendation [G.983] is established by an institution of ITU-T.
As shown in FIG. 61, in ordinary case, the ATM-PON (communication system) 100 is arranged to include a single unit of station-side unit (OLT: Optical Line Termination) 101 and N sets of subscriber-side units (N is an integer larger than one. In accordance with the recommendation [G.983], N is 64 at maximum) (ONU: Optical Network Unit) 103 connected to (accommodated in ) the station-side unit by way of a passive device 102 such as an optical coupler. Thus, the communication network is arranged as a one-to-N connection topology. Although not illustrated in FIG. 61, each ONU 103 accommodates terminals of respective homes and the OLT 101 is housed within a station of an ATM switching system or the like. Thus, an optical communication is effected up to the ONU 103 through the optical fibers.
In the network of the ATM-PON 100, an ATM cell on the downstream sent from the OLT 101 is branched by the passive device 102 in a passive manner, whereby identical ATM cells are distributed to all of the ONUs 103 (in a point-to-multipoint communication fashion). Conversely, an ATM cell on the upstream sent from one of the ONU 103 is transmitted through the passive device 102 and received by the OLT 101 in a time series fashion. When such an upstream transmission is carried out, however, if a plurality of ATM cells are sent from any of the ONUs 103 at the same timing, collision will be brought about between cells. Therefore, the OLT 101 controls (ranging time control) each of the ONUs 103 so that the timings when cells are sent therefrom are shifted from one another.
The ATM-PON 100 transmits and receives a management cell known as a PLOAM (Physical Layer Operation, Administration and Maintenance) cell between the OLT 101 and ONUs 103, whereby various kinds of communication control such as the aforesaid ranging time control, maintenance and administration are carried out.
That is, since the PLOAM cell is arranged to contain desired control information such as the ranging time information and information (message) regarding the maintenance and administration, various communication control and management information can be exchanged between the OLT 101 and the ONUs 103. In this case, the units of OLT 101 and ONUs 103 may be manufactured by an identical vendor. Conversely, as shown in FIG. 61, these units may be manufactured by different vendors (company A, company B, and soon). In any case, the above-described minimum requirement in function is supported in accordance with the recommendation [G.983].
Meanwhile, the above recommendation [G.983] contains a regulation regarding away of using the aforesaid messages. One of the massages is a message specific to a vendor (VSM: Vendor Specific Message). However, how to utilize the VSM is not specified in the recommendation [G.983]. In other words, the message specific to the vendor is allowed to utilize (define) freely depending on the vendor. For this reason, it is considered to carry out an information supplying operation such as a software download (SWDL) in which a desired software is supplied from the OLT 101 to the ONU 103 by using the VSM, for example.
In more concretely, according to the recommendation [G.983], the VSM has a format shown in FIGS. 62A and 62B [in this case, FIG. 62A shows a format of the VSM for use in downstream transmission (OLT 101 to ONU 103) while FIG. 62B shows a format of the VSM for use in upstream transmission (ONU 103 to OLT 101)]. The VSM cell (total 53 bytes) for the downstream transmission has a field of forty-second to fifty-first byte (ten bytes in total) from the head thereof as a message field 111 which can be utilized freely by each vendor. The VSM cell for the upstream transmission similarly has a field of ninth to eighteenth byte (ten bytes in total) as a message field 111 which can be utilized freely by each vendor. The VSM cell for the downstream transmission and the VSM cell for the upstream transmission have a message ID field 112 at the forty-first byte and eighth byte, respectively. A value of “01111zzz” [lower three bits (z) can take an arbitrary number] is attached on the field so as to indicate that the subject message is a message of VSM.
In other words, according to the recommendation [G.983], eight kinds of message IDs of from “01111000(0×78H)” to “011111111(0×7fH)” are prepared as the message ID for the VSM.
The OLT 101 attaches a PON-ID (identification information specific to each of the ONUs 103) or a PON-ID(=01000000(0×40H)) indicative of a broadcast type communication (point-to-multipoint communication) to a downstream VSM message at fortieth byte (PON-ID field 113), whereby a specified one (single unit) of the ONUs 103 or all of the ONUs 103 are brought into a message receiving mode.
As for example shown in FIG. 63, if the PON-ID indicative of the broadcast type communication is attached to the downstream VSM (PLOAM cell) (see reference {circle around (1)} all of the ONUs 103 receive the VSM. Conversely, if the PON-ID having a unit-specifying function is attached to the downstream cell, only an ONU 103 having the PON-ID stored therein can receive the VSM, and other ONUs 103 (i.e., ONU 103 having a different PON-ID) will discard the received VSM (see references {circle around (2)} to {circle around (4)}).
According to the arrangement of the ATM-PON 100 shown in FIG. 61, a main signal generated from the OLT 101 is distributed through the passive device 101 in a passive manner. Therefore, it is physically impossible to distribute the signal (message) from the OLT 101 to a selected one of ONUs 103. Accordingly, all of the ONUs 103 once receive the same signal and each of the units 103 determines whether the received signal shall be discarded or not depending on whether or not the PON-ID attached to the received signal (at the PON-ID field 113) is coincident with its own identification number. In this way, the signal can be selectively received.
Meanwhile, according to the recommendation [G.983], the following is forcibly recommended. That is, when the OLT 2 assigns an individual PON-ID number to each of the ONUs 103, the ONU 103 sends to the OLT 101 a serial number (Serial Number) ONU message 109 as shown in FIG. 64 by using the upstream PLOAM cell.
In this case, as shown in FIG. 64, when the message is formed into a PLOAM cell, the serial number ONU message 109 comes to have a format which is defined in such a manner that a PON-ID field 113 is mapped at a seventh byte from the head of the cell (second byte from the payload), a message ID field 112 is mapped at eighth byte of the head of the cell, a vendor ID field 110a is mapped at tenth to thirteenth bytes from the head of the cell, and a vendor serial number (Vendor Serial Number) field 110b is mapped at fourteenth to seventeenth bytes from the head of the cell. The format of the cell has a fixed value (00000000) mapped on a field at a ninth byte from the head of the cell. However, this field has no particular function. Also, a field extending from a eighteenth byte from the head of the cell is an unspecified byte.
The ONU 103 settles the vendor ID in the vendor ID field 110a and a serial number for each vendor in the vendor serial number field 110b. Thus, a serial number information specific to each ONU 103 composed of the vendor ID of its own unit and the serial number for each vendor is stored in the serial number field 110, and the OLT 2 is informed of the serial number.
At this time, a fixed value of “01000000” (0×40H) is settled in the PON-ID field 113 (after the individual PON-ID has been assigned to each unit, the PON-ID is settled). Further, a value of (00000011) indicating that the message itself is the serial number ONU message 109 is stored in the message ID field 112.
If the OLT 101 receives the serial number ONU message 109 from each of the ONUs 103 and holds therein information indicative of the correspondence between the serial number involved in the message 109 and the PON-ID to be assigned to each ONU 103, it becomes possible to assign the individual PON-ID to each ONU 103, freely, and also it becomes possible to specify the vendor of the ONU 103 based on the individual PON-ID assigned to the ONU 103.
In the above-arrangement of the ATM-PON 100, however, when a message is sent to the ONU 103, the OLT 101 suffers from a restriction in the manner of transmission. That is, as described above, a message is allowed send to all of the ONUs 103 as a broadcast type communication, or alternatively, a message is allowed to send to only a single unit of ONU 103 (point-to-point communication). Therefore, the arrangement will encounter the following difficulties.
(1) According to the recommendation [G.983], it is not guaranteed that the VSM can be exchanged among units in a compatible fashion whichever vendor the units are manufactured by. For this reason, if a VSM is sent to the units in a broadcast communication manner, an erroneous operation can be caused at a high possibility in an ONU 103 of which vendor is different from that of the OLT 101 upon receiving the VSM.
(2) When it is requested that a VSM is exclusively sent to ONUs 103 of which vendor is a particular one, such as when a software such as the aforesaid SWDL is distributed to the ONUs 103 which are manufactured by an identical vendor and commonly necessitate the software, there is no choice in the way of communication but the point-to-point communication due to the difficulty identified by the above Reference (1). In this case, the PLOAM cell (VSM) having the same contents shall be transmitted repeatedly by a number of times corresponding to the number of ONUs 103 as a target of transmission. Therefore, the number of output messages from the OLT 101 is increased, a heavy load is imposed on the OLT 101 and the ONUs 103, it takes a lot of time to download the message, and steep rise will be caused in the network traffic.
In other words, according to the arrangement of conventional ATM-PON 100, the PLOAM cell (message) can be transmitted only in manners of broadcast type communication and point-to-point communication. Therefore, it is impossible to send a VSM to a particular plural number of ONUs 103 exclusively in a manner of point-to-multipoint communication. Thus, the above-described difficulties are caused.
The present invention is made in view of the above aspect. Therefore, it is an object of the present invention to provide a communication system in which a station-side unit can send a message only to a particular plural number of subscriber-side units in a manner of point-to-multipoint communication. Also, an object of the present invention is to provide a method of processing the message in the communication system and a station-side unit and a subscriber-side unit for use in the communication system.